1. Field of the Invention
The present invention relates to a back-side illumination sensor, and more particularly, to a back-side illumination sensor with a reduced area.
2. Description of the Prior Art
As the pixel size of a complementary metal-oxide-semiconductor image sensor (CMOS image sensor, CIS) grows smaller, the degradation resulting from certain factors such as quantum efficiency, cross-talk and so forth in a sensor array also becomes significant. Regarding a conventional image sensor such as a front-side illuminated sensor, a lens of each pixel sensor is fabricated on a front side of a substrate. Therefore, the incident light has to travel through dielectric layers between circuitry formed by metal layers to arrive at a photo diode otherwise it will be reflected or absorbed by metal or any other reflective material.
In order to enhance the compactness of an image sensor, a back-side illumination (BSI) image sensor is introduced. In a BSI image sensor, the incident light is projected from a back side of a substrate of the image sensor, and therefore the front side of the substrate can be reserved for circuitry of various functions. Please refer to FIG. 1, which is a partial cross-section view of a pixel structure of a conventional BSI image sensor. The pixel for includes a photodiode 100, a transfer transistor 200, a reset transistor 500, and an isolation structure 400. The photo diode 100 is composed of an N-doped layer NL and a P-doped layer PL, and transfers incident light into electrical signals. The transfer transistor 200 (in this example, an N-type metal-oxide-silicon transistor fabricated on a P well) thereby transmits the electrical signals from the photo diode 100 into other circuits for subsequent processing via an N-doped connection path 300 according to a control instruction received at a gate electrode. The isolation structure 400 is fabricated herein to prevent noise from corrupting an integrity of the electrical signals. The reset transistor 500 is used for resetting the photodiode 100.
With the utilization of the BSI technology, there are more and more available areas on the front side of the substrate for building various functional circuits. To further exploit advantages of the BSI structure, however, more and more effort is dedicated to improve an area efficiency and compactness of a BSI image sensor.